IK) PROCEEDINGS OF THE SOCIETY. 



T. tiihf'rflSiHm n precipitation of the cell-contents was first observed 

 which blocked the cord conveying the nutrition to the margin. An 

 investigation by trained microscopists would undoubtedly bring most 

 valuable information, and would advance our knowledge of the photo- 

 synthetic action induced in living cells and their products. 



Mr. Scourfield and Dr. Rudd Leeson commented upon some of the 

 points raised by Col, Rawson, 



Mr. F. I. G. Rawlins made a short communication on the Techni([ue 

 of the Vertical Illuminator, in which he pointed out that from experience 

 in metallography it had been found unnecessary to use objectives in 

 special short mounts with the vertical illuminator up to and including 

 ^}th powers, provided the objectives were corrected for work on un- 

 covered objects. Obviously, this was an advantage in war time, when 

 specially mounted lenses were unobtainable. It was quite possible to use 

 the illuminator diaphragmed down to its smallest aperture, and so 

 increase the definition. An illuminant such as the electric arc, giving 

 light of short wave-lengths of considerable energy within the visible 

 spectrum, was preferable for this work. Great import<ince was attached 

 to the levelling of specimens, since failure to achieve this threw a large 

 part of the reflected light outside the tube of the microscope. Level- 

 ling was best done by pressing the specimen face downwards on to a 

 piece of plate-glass with a small quantity of plasticine on a common 

 3 in. by 1 in. slip. The latter was rested on the two edges of an 

 accurately cut ring, and held there until the preparation had become 

 embedded in the plasticine. For preserving metal specimens, a thin 

 coating of a concentrated solution of gun-cotton in amyl-acetate was 

 recommended — a method originally due to Professor Le Chatelier — as 

 a preventive against rust. This was dropped on to the surface, and 

 the section tinted until 'the drop found its own level, and set quite 

 evenly to a thin layer, sufficiently transparent for use with a J.th objec- 

 tive. The varnish must not be applied with a brush, or ridges resulted 

 which gave brilliant interference colours when viewed under the micro- 

 scope, . In any case it was important to line the inside of the tube 

 with black velvet (such as was done in microphotography), to lessen 

 the reflection as the light returned from the specimen to the observer, 

 and in the cover-glass pattern illuminator to obviate the flare caused 

 by that portion of the light which did not suffer reflection at the surface 

 of the glass disc of the illuminator, but which penetrated the glass and 

 struck the opposite wall of the tube. 



Fellows were referred to a paper in the " Philosophical Magazine " 

 for November, 1917, for employment of the vertical illuminator for 

 biological work, where Lord Rayleigh dealt with the optical properties 

 of collodion films (suggesting a possible analogy with a cover-glass). 



